This invention relates to a method of preparation of a dilute standard gas, said gas itself and an apparatus for the performance of said method.
Many difficulties have been encountered in conventional methods for preparation of a dilute standard gases used, for example, in the calibration of standards in gas microanalysis and also for the calibration of automatic, continuous gas analyzers.
Among the difficulties encountered in conventional methods there may be mentioned inevitable errors resulting from unrepresentative sampling of a given gas, unhomogeneous mixing of said gas with a diluent, loss of gas by the absorption thereof onto the surface of receiver vessel and also a change of concentration of gas due to the generation of the target (desired) gas itself or due to the instability of the gas.
Among the many methods which have been hitherto proposed to provide accurate, dilute standard gases, the permeation tube method is considered to be most superior and widely used. The permeation tube consists of a tube of a polymeric substance such as Teflon containing a liquified or a highly compressed gas to be used in the preparation of the standard gas.
Both ends of the permeation tube are sealed. Since the vapor pressure of the gaseous sample in the inside of the tube becomes high, a small amount of the gas is always purged through the wall of the tube. If the temperature of the tube is constant, the amount of purged gas per unit time becomes constant. The permeation tube is then placed in a fresh air flow of a definite flow rate at a constant temperature to provide a dilute gas of definite concentration.
The permeation tube method has advantages such as the fact that it is possible to directly generate a very small amount of gas at a constant rate and moreover it is, of course, possible to determine the absolute rate of gas generation from the permeation tube, merely by measuring the weight loss of the tube.
On the other hand, the permeation method has several disadvantages in practice, although the basic principle of the method is rather simple. The main disadvantages include the necessity to carefully regulate the temperature of the air flow since the rate of generation of gas from the tube changes drastically with a small change of the temperature, a rather long time lag to obtain a gas flow of a constant concentration after the beginning of the flow of air and the need for skill in the preparation of the permeation tube, despite the simplicity of the principle.